1. Filed of the Invention
The present invention relates to an image reading apparatus which reads a document, and to a copying machine having the image reading apparatus.
2. Discussion of Background
Recently, protection of the environment and saving energy are demanded, and standards for the purpose of saving energy such as Energy Star and ZESM etc. have been established in a field of a copying machine etc. Those standards provide limitation of consumption of energy during waiting time. In response to those standards, most of copying machines are designed such that during the waiting time, power sources of a fixing heater and an operation panel etc., whose consumption power is relatively large, are switched off or switched to a low power operation. In an image reading apparatus installed in a copying machine, it is general rule that power sources of all devices are turned off.
When using a copying machine which is in a state of the power being turned off or in the low power operation, after the power is turned on, a user is forced to wait until the copying machine becomes usable. At this time, in order to turn the copying machine in a usable state, in the image reading apparatus installed in the copying machine, generally, the following adjustments are necessary;    Adjustment of amplifying degree for amplifying an analog image signal at proper large amplitude    Adjustment of offset level for making black level at optimum value
In the technique disclosed in Japanese Laid Open-Patent Publication No. 6-326867, in an image reading apparatus, the above adjustments are executed when the power is turned on so that a read analog image signal can be accurately converted to a digital image signal.
Further, in the adjustment of the offset level, data of a predetermined number of lines is read while changing a setting value for adjustment, and an offset setting value which is optimum is determined by calculation formula, and then the offset level is adjusted.
In the technique which adjusts the offset level as described above, because only data of limited predetermined number of lines is read, the adjustment time is short. However, on the other hand, because the offset value is sought indirectly from the calculation formula, there is a problem that a slight setting error occurs.